1. Field of the Invention
This invention relates to a control system for use with internal combustion engines and, more particularly, to a system including a microcomputer for processing information on various engine operating conditions.
2. Description of the Prior Art
There have been developed systems using a digital microcomputer for controlling various engine operating parameters such as the amount of fuel supplied for an engine, spark timing, the rate of exhaust gas recirculation. Such a control system is fed with information on various engine operating conditions in the form of analog signals representative of engine rotational speed, intake air flow rate, engine temperature, and the like. These analog signals are converted into digital signals for application to the microprocessor. The control system also receives on-off signals from various switches such as a throttle switch adapated to detect the fully closed position of the throttle valve, a neutral switch adapted to detect the neutral position of the power transmission, a starter switch adapted to detect the cranking operation of the engine, a switch adapted to detect the actuation of an air conditioner, and the like. These on-off signals are applied as interrupt signals to the microprocessor.
As an example, when the signal from the throttle switch changes to its off state, the engine is accelerating. The throttle switch is adapted to provide an ON signal at the fully closed position of the throttle valve and an OFF signal at the open position of the throttle valve. This signal change is used as an interrupt signal to cause a temporary discontinuation in a routine being performed by the microprocessor; and directs the microprocessor to enter to another routine for making corrections required during acceleration. At the end of the routine, the interrupted routine is resumed from the point at which the discontinuation occurred.
In such a conventional system where switch signals are used as interrupt signals a simple input circuit may be utilized. The system operates at a rapid response rate with respect to engine operating condition changes, but requires complicated hardware and software systems to give priority levels to the incoming switch signals as the number of the associated switches increases. In addition, due to an instantaneous change in the state of a switch which requires no actual interrupt of the program sequence, an unnecessary signal interrupt may occur to disturb other routines. For example, when the gear position of the transmission is changed, the throttle switch comes on and returns to its off state in a short time. Thus, each time the transmission is changed to another gear position, an interrupt occurs to cause entry into a routine required in acceleration such as to increase the amount of fuel supplied to the engine. When the gear position of the transmission is changed, however, this acceleration routine is unnecessary and spoils exhaust gas purifying performance.